William served during the US Civil War as a private in Company "F", 16th New York Cavalry and Company "B", 3rd New York Provisional Cavalry. His widow received a pension as an Army Widow.
From the Military Record of Huron County: William F. Barnum was eighteen years of age when he volunteered, enlisting at Potsdam, New York, July 21, 1863, and at Plattsburg, New York, mustering into Company F, 16th New York Cavalry. The regiment went into service assigned to Lowell's Brigade, Tyler's Division, 22nd Army Corps, and the battles in which Mr. Barnum took a soldier's part include Cedar Creek, second engagement at Culpeper Court House, the Wilderness, and many other engagements of Grant's great closing campaign, the brigade being engaged in scouting duty most of the time. The regiment was badly cut up in service, and was consolidated with the 13th New York Cavalry, the organization known as the 3rd New York Provisional Cavalry. From a total of 1,350 men the 16th had lost about one thousand. Some of its marching is the most noted recorded of the war, at one time, contending with Moseby, 150 miles were marched in forty-two hours. After an experience of two years and two months in such service as this Mr. Barnum received his discharge. He was mustered out at Washington City, September 21, 1865, and received discharge at Harts Island, New York, October 5, 1865. Buel P. and Cornelia A. (Northup) Barnum are the parents of William F., who was born in Potsdam, New York, but has lived in Huron county, Ohio, since he was eleven years of age. His first marriage was with Dolly E., daughter of Major and Eliza (Knapp) Smith. She was born in Ohio and they were married in Clarksfield, Huron county, June 2, 1866. Their children were two sons: Charles P., born August 4,1868; Jay M., August 27, 1870. In Clarksfield, September 29, 1875, Kate E. Young become the wife of Mr. Barnum. She was born in New York, daughter of Willis and Mary E. (Smith) Young. Mr. and Mrs. Barnum have one daughter, Alice May, born August 27, 1878. He is engaged in an insurance business at North Fairfield, and is commander of Todd Lee Post, No. 503, G. A. R.